Transient voltage surge suppressor

ABSTRACT

A transient voltage surge suppressor is provided to protect equipment with voltage sensitive electronics, such as office equipment, from surges or transients in the multi-phase power distribution network furnishing power. The surge suppressor senses the incoming voltage and electrically disconnects the neutral leads and hot leads of the power network from the neutral lead and hot lead of the office equipment if a voltage surge above an established level is sensed. The suppressor also disconnects its output when a ground connection in the multi-phase power network is disconnected to protect the equipment against excessive voltages. In this situation, the suppressor circuit protects its internal components from damage and possible fire hazard when its output is disabled, so that the suppressor can resume its function when conditions in the power distribution network return to a satisfactory state.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of prior co-pendingU.S. patent application No. 09/311,240 filed May 13, 1999 now U.S. Pat.No. 6,229,682.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to electrical protective filters ortransient voltage surge suppressors (TVSS) for office equipment or anymicroprocessor-based equipment. These suppressors provide surgefiltration to protect the office equipment from surges or transients inthe power furnished from the power distribution network.

2. Description of the Prior Art

Numerous types of office equipment used in recent years containelectronic chips or components which are sensitive to voltage surges ortransients in the power distribution network provided by utilitycompanies. Examples of such types of office equipment include computers,facsimile machines, photocopiers and the like. Devices known as powerfilters or transient voltage surge suppressors have been developed toprotect these types of office equipment from such power surges ortransients. U.S. Pat. No. 5,721,661, of which applicant is inventor, isone form of such a power filter.

Office buildings and other places where such equipment has been usedhave often been based on a three-phase or center-tap system of powerdistribution. There are several advantages from the use of this form ofpower. It allows more power to be delivered to a site, is more easilydistributed, and allows two voltage levels (such as 120 volts and 240volts) to be delivered. Customers can thus use equipment requiringlarger amounts of power. These forms of power are a more cost effectiveway for an electric utility to transmit power to an end user. In threephase or center tap power, in the event of loss of the ground wire orneutral wire connection in the distribution grid, voltage levelsprovided in the network could increase up to a double level. This is notan uncommon event. Equipment rated at 120 volts could thus be subjectedto up to 240 volts. So it could possibly burn out a surge protectioncomponent and possibly cause firing the TVSS unit, which could also burnout the connected equipment.

So far as is known, it was typical to use relatively inexpensive thermalfuses for protective purposes in these situations. These fuses wouldtypically open after response to excess heat for periods of from coupleof seconds to several minutes. It may protect the TVSS against firingafter burn out, but sensitive electronic chips and circuits of connectedequipment were not capable of withstanding such excessive voltage levelsfor even short fractions of a second, such as a few milliseconds.

SUMMARY OF INVENTION

Briefly, the present invention provides a new and improved protectivecircuit for electrical apparatus. The protective circuit has hot,neutral and ground leads which are arranged to be placed betweencorresponding hot, neutral and ground leads of the electrical circuit.The protective circuit includes a voltage surge protection andfiltration circuit connected between the hot and neutral leads, alsobetween neutral and ground, and also between the hot lead and ground.

The voltage surge protection circuit has first and second voltage surgeprotectors connected in an electrical series relationship with eachother. The first and second voltage surge protectors are connected at acommon connection or node to each other. The first and second voltagesurge protectors are connected between the neutral lead and ground leadto suppress and filter neutral ground noise and transients. If desired,a second set of first and second voltage surge protectors, similarlyconnected to each other and connected to the first set through a noiseisolation transformer or differential transformer may be included.

The protective circuit also includes a series-connected set of first andsecond relays connected between the hot and neutral, as well as hot andground, leads of the protective circuit. The first relay has a switcharm to connect the clamping device, such as an MOV gas tube, Zener orany other type of voltage clamping component, also to connect afiltration component such as capacitor or inductor between the neutrallead and ground lead to suppress and filter the neutral ground (commonmode) noises and spikes when the first relay is receiving current. Thefirst relay is typically receiving current when the ground lead isproperly connected to ground, also the hot and neutral leads arecorrectly connected to hot and neutral, not reverse polarity.

The first relay switch arm opens when current in the first relay circuitis not being received or present, such as when the ground connection ofthe ground lead is interrupted or broken. Opening the first relay switcharm isolates the electrical apparatus from the power utility. It alsoprotects the voltage surge protectors and filtration components. Theprotective circuit of the present invention thus protects the voltagesurge protectors and filtration components such as MOV's and capacitorsand any type of voltage protector devices (such as a gas-tube, ZENER orthe like) between neutral and ground or hot to ground in the voltagesurge protector circuit. Protection is provided against over voltage(240 V) when connection to an electrical ground is broken or reversepolarity of hot and neutral causing voltage increase up to 240 V.

The second relay of the protective circuit also controls a switch armwhich is located in a conductor connecting the hot lead of the powerutility outlet to the hot lead of the electrical apparatus when thesecond relay is receiving current. The second relay switch arm openswhen current in the second relay is not present, such as when the groundconnection of the ground lead is interrupted or a reverse polarity ofhot and neutral or when the voltage increases up to 240 V.

The protective circuit of the present invention also protects allcomponents between hot and neutral and thus protects the electricalapparatus when ground connection to electrical ground is broken orinterrupted. Protection occurs against reverse polarity of hot andneutral or when voltage increases up to 240 V, and also in the event ofapplying 240 volts to hot and neutral without ground connection.

The protective circuit of the present invention also includes a voltagethreshold sensing circuit which detects when the voltage between the hotand neutral lead of the power utility outlet exceeds an establishedprotective level. An electronic switch is connected to the voltagethreshold sensing circuit. The electronic switch responds to power fromthe utility outlet detected as exceeding the threshold and disables thesecond relay which supplies current to the connected apparatus ormicroprocessor-based equipment. The electrical apparatus is thusprotected by the circuit of the present invention from excess voltagelevels between the hot and neutral lead from the power utility outlet.An indicator, such as an LED, also turns on to indicate the abnormalityof voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic electrical circuit diagram of a transient voltagesurge suppressor or transformer base filter according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the drawings, the letter P designates generally a new and improvedprotective circuit or transient voltage suppressor for electricalapparatus. The protective circuit P functions as a transformer basefilter. In the preferred embodiment, the electrical apparatus may takethe form of a computer, a copier, a facsimile machine or the like havingvoltage surge or transient sensitive electronic components, such ascomputer chips. These types of components are sensitive to voltagesurges or transients, as well as abnormal high voltages furnished themby leads from electrical power utilities. It should be understood,however, that the protective circuit P may be used with other electricalapparatus which are sensitive to voltage surges or transients, or toabnormal high voltage.

The protective or transformer-based filter circuit P has as its inputs ahot lead or terminal 10, a neutral lead 12, and ground lead 14, each ofwhich is connected respectively to a hot lead 16, a neutral lead 18 anda ground lead 20 of a conventional electrical power utility outlet U.Typically, the embodiment shown in the power provided is conventional120 volts alternating current potential difference between the hot lead16 and the neutral lead 18. It should be understood that the voltagelevel present could be some other conventional or standard voltagelevel.

The protective circuit P includes a voltage surge protective circuit Vconnected between a hot conductor 10 a connected to the hot terminal 10and neutral conductor 12 a connected to the neutral lead 12. The voltagesurge protective circuit V has a first set of voltage surge protectivedevices or clamping devices 24 and 26 and a second set of voltage surgeprotective devices, such as filters or clamps 28 and 30 and also acapacitor 34. Each of the two sets of voltage surge protective devicesis connected in an electrical series circuit between the hot and neutralleads 10 and 12.

The series connected clamping devices 24 and 26 of the first set areconnected at a node or common connection 32 to each other opposite theirrespective connections to the hot and neutral leads 10 and 12.Similarly, the series connected clamping devices 28 and 30 of the secondset of the circuit V and also the capacitor 34 are connected to eachother at the node 32 and thus to the clamping devices 24 and 26 of thefirst set. The node 32 is connected to the neutral lead 12 through anoise filter capacitor 34. The noise filter capacitor 34 may be anyrating or capacitance value, such as 2 μf or larger. A current limitingresistor 36 and a second noise filter capacitor 38 are also provided inparallel with capacitor 34 to connect the node 32 to the neutral lead12.

The protective circuit P also includes a relay circuit R composed of aset of a first relay 40 and a second relay 42 connected between the hotand neutral leads 10 and 12. As is conventional, a protective diode 44and a filter capacitor 46 are connected in parallel across the relays 40and 42.

As will be set forth, the relay set R protects against severalundesirable conditions. The relay set electrically R isolates theelectrical apparatus connected to the protective circuit P fromtransients and protects against over voltage or other undesirableconditions occurring in the power utility. The first relay 40 and secondrelay 42 protect voltage surge protective devices 24, 26, 28, and 30,and capacitor 34 when ground lead 14 is not connected to building ground20. The first relay 40 and the second relay 42 also protect thesevoltage surge protective devices against high voltages between hot lead10 and ground lead 14, and also high voltages between neutral lead 12and ground lead 14, such as those occurring when connection to buildingground 20 is broken, reverse polarity between hot lead 10 a and neutrallead 12 a, or disconnection of the neutral lead 12 a.

In addition, the first relay 40 and the second relay 42 protect voltagesurge protective devices 43 and 45 between hot lead 10 and neutral lead12 against high voltages. The first relay 40 and the second relay 42 aswell protect the electrical apparatus at the output of protectivecircuit P against high voltages. As will be set forth, the first relay40 and the second relay 42 also protect the other voltage surgeprotectors and electrical filter components between the protectivecircuit P against such undesirable conditions.

The first relay 40 has a relay coil 40 r controlling the position of aswitch arm or contact 40 k. The contact 40 k of the relay 40 iselectrically connected in the neutral conductor 12 a between the neutrallead 18 of the power utility and the neutral lead 12 b to the electricalapparatus. As long as the electric ground 14 a is electrically connectedto ground, and also, as long as the connection between the hot andneutral leads 10 and 12 is not reversed, the relay 40 receives currentand relay contact 40 k is in a closed position shown in the drawing.

The first relay switch arm 40 k opens when current is not present in thefirst relay 40 in the event of the undesirable conditions mentionedabove. The protective circuit P of the present invention thus protectsits voltage surge protectors or clamping devices, as well as itselectrical filter elements, as well as any devices between neutral andground. In addition to the protection provided against these undesirableconditions, the protective circuit P protects and isolates an electricalapparatus which is connected to it from undesirable conditions presenton lines from the electrical utility.

When the ground wire 14 a is properly connected also the hot and neutralwires are properly connected, then a thyristor 70 is on. The relays 40turns on and the switch 40 k is in the position shown in the drawings,and first relay arm 40 k closes the circuit between the neutral lead 12a. In this manner any power surges or transients between the hot lead 10and neutral lead 12 are suppressed by the voltage protective devices 24and 26, also by devices 28 and 30 when present. Further, noise andtransients also are filtered by capacitors 34 and 38. In this manner,substantially all electrical noise which may from time to time appearbetween the hot or neutral leads 10 and 12 is filtered.

In the event an electrical connection between the ground lead 14 andbuilding ground 20 is interrupted, or if the connection between the hotlead 10 and neutral lead 12 should become improperly connected andpolarity thus reversed, or if the neutral lead connection 12 isinterrupted or broken, an abnormal situation is present. The voltagelevels present can increase to twice their normal level. The relay 40 nolonger receives current through thyristor 70 or other electronic switch,such as a transistor. In such a case, the contact 40 k of the relay 40opens and moves to an opposite position from that shown. Current cannotflow through it to voltage clamping devices 24, 26, 28 and 30 or thenoise filter capacitors 34 and 38. Thus, the clamping components of thevoltage surge protection circuit V are protected from increased voltage.

Winding bobbins 41 a and 41 b of a differential transformer (or noiseisolation transformer) 41 are series connected between hot lead 10 a and10 b and between neutral 12 a and 12 b, respectively. The noiseisolation transformer 41 combines with capacitor 34 to filter the noisesbetween the hot and ground or neutral lead and ground 20.

A voltage clamping device 43 is provided to suppress any surges ortransient between hot lead 10 a and neutral lead 12 a and a clampingdevice 45 suppresses any surges and transients between hot lead 10 b andneutral lead 12 b. Capacitors 47 and 49 are furnished for filtering anynoise between hot lead 10 b and neutral lead 12 b while a resistor 51discharges the capacitors 47 and 49.

The second relay 42 of the relay circuit R includes a coil 42 r whichcontrols a switch arm or contact 42 k in hot conductor 10 a connectingthe hot lead 16 of the electrical power utility outlet U to a hot leadoutput 10 b connected to the electrical apparatus. So long as the relay42 is receiving electrical current, the contact 42 k is closed (as isshown) and electrical power is furnished at the hot lead output 10 b tothe electrical apparatus. In the event that current through the relay 42is no longer present, such as when the ground connection is interrupted,or polarity between the hot lead 10 and neutral lead 12 becomesreversed, the contact 42 k of the relay 42 opens. Power is thusinhibited from being present at hot lead 10 b, and the electricalapparatus connected there is protected from voltage surges, over voltageor transients. Also any clamping components between hot lead 10 b andneutral lead 12 b are protected against increased voltage.

The protective circuit P of the present invention also includes avoltage threshold sensing circuit T which detects when the voltage onthe hot lead 16 of the electrical power utility outlet U exceeds a setor established voltage threshold. The voltage threshold is establishedby the relative impedance values of series connective resistors 50 and52. The voltage threshold may be adjusted by selecting differentimpedance values for the two resistors, or by including a variableresistor or rheostat in one or both of the resistors 50 or 52. A DCfilter capacitor 54 is connected in parallel with the resistor 50. Aclamping device 55 is connected between leads 10 a and 12 a to limit thevoltage drop across the voltage threshold sensing circuit T. Theresistor 52 is electrically connected to the neutral conductor 12 a andneutral lead 12 by a resistor 56. A diode 58 is present to rectify theAC voltage to DC. A protective fuse 59 may also be provided in lead 10 aat the input of voltage threshold sensing circuit T.

A Zener diode 60 is electrically connected to resistors 50 and 52 tosense the voltage threshold level present there. As long as the voltagethreshold does not exceed the established threshold, the diode 60 doesnot conduct. In the event that the voltage threshold level provided todiode 60 exceeds the set level, diode 60 begins to conduct and atransistor or other electrical or electronic switch 62 also begins toconduct. The transistor 62 is normally held in a non-conductive state.

A collector terminal 62 c of the transistor 62 is connected to a gate 70g of a thyristor 70, or other suitable electronic switch, such as atransistor, connected in series with the relays 40 and 42. When thetransistor 62 begins to conduct due to the threshold voltage level beingexceeded, the current through diode 64 and resistor 66 to the gate 70 gof the thyristor 70 is drawn to zero. Thyristor 70 is then switched to anon-conductor or off state and current flow through relays 40 and 42 isterminated. In this manner, relays 40 and 42 of the voltage surgeprotective circuit V are disabled in the event of excess voltage betweenhot lead 16 and neutral lead 18, protecting the circuit P and connectedapparatus from excessive input voltage.

Thyristor 70 is normally biased to a conductive state and provideselectrical current to relays 40 and 42 by a bias network including diode64 and resistors 66 and 74. A diode 78 rectifies the AC to DC andresistor 76 limits the current in relay coils when the thyristor 70 isconductive and thus relays 40 and 42 are receiving current. An alarmindicator 80, such an LED, is connected through a diode 84 and aresistor 86 to the neutral lead 12 a. The alarm indicator 80 isenergized when the relay switch 42 k is in the open position oppositethat shown in the drawings. In the event that contact 42 k of relay 42interrupts the flow of power to the electrical apparatus via the hotconductor 10 a, indicator 80 is electrically energized to indicate thisalarm condition.

A normal indicator 90, such as an LED, is connected between the hot lead10 b and the neutral lead 12 b in series with a diode 92 and a resistor94. During normal operation of the protective circuit P when the relays40 and 42 are receiving current, the relay switch 42 k is in theposition shown in the drawing connecting the hot lead 10 a through tothe hot lead 10 b. Relay switch 40 k is also in the position shown inthe drawing connecting the neutral lead 12 a through neutral lead 12 b.In this condition, the normal indicator 90 is energized, indicating thatconditions are normal for power flow through the protective circuit P tothe electrical apparatus.

It is to be noted that each of the various voltage clamping devices, aswell as the electrical filter elements, such as capacitors andinductors, in the protective circuit P are connected between the neutrallead and the ground lead. In this manner, an undesirable effect, such asneutral ground (or common mode) noise and transient spikes are filteredand suppressed by the protective circuit P.

It should be understood also that the relays 40 and 42 may be separatelyconnected individually between the hot lead 10 a and neutral lead 12 a,rather than in series as shown in the drawing. In this separateconnection, each of the relays 40 and 42 are provided with a separatethyristor or other electronic switch functioning like the thyristor 70.Each such separate thyristor or electronic switch is of course providedwith its own corresponding bias network of the type described above.Also, if desired, one relay with two sets of switch arm contacts may beused as an alternate.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

What is claimed is:
 1. A protective circuit having hot, neutral, andground leads arranged to be placed between corresponding hot, neutral,and ground leads of a power utility outlet and corresponding hot,neutral, and ground leads of electrical apparatus, said protectivecircuit comprising: a voltage surge protection circuit connected betweenthe hot and neutral leads, comprising: first and second voltage surgeprotectors connected in series between the hot and neutral leads, saidfirst and second voltage surge protectors being connected at a commonconnection to each other; a first relay connected between the hot andneutral leads of the protective circuit and controlling a first relayswitch arm which connects the neutral lead of the power utility outletand the neutral lead to the electrical apparatus when the first relay isreceiving current, said first relay receiving current when the groundlead is connected to an electrical ground, and said first relay switcharm opening when current in the first relay circuit is not received toprotect the electrical apparatus and the voltage surge protectors of thevoltage surge protector circuit when the ground lead is not connected toan electrical ground.
 2. The protective circuit of claim 1, furtherincluding: a capacitor connected between said common connection of saidfirst and second voltage protectors in said voltage surge protectioncircuit and said neutral lead, said capacitor having a farad rating tofilter substantially all noise signals on said neutral lead to groundwhen said ground lead of said circuit is connected to electrical groundthrough said first relay switch arm.
 3. The protective circuit of claim2, further including: a voltage threshold sensing circuit for detectingwhen the voltage on the hot lead of the power utility outlet exceeds anestablished protective level; a relay supply switch for providingcurrent to the relay circuit; and an electronic switch responsive to thevoltage threshold sensing circuit for disabling the relay supply switchand protecting the electrical apparatus when the voltage between the hotlead and the neutral lead of the power utility outlet exceeds anestablished protective level.
 4. The protective circuit of claim 1,further including: a voltage threshold sensing circuit for detectingwhen the voltage between the hot lead and the neutral lead of the powerutility outlet exceeds an established protective level; a relay supplyswitch for providing current to the relay circuit; and an electronicswitch responsive to the voltage threshold sensing circuit for disablingthe relay supply switch and protecting the electrical apparatus when thevoltage between the hot lead and the neutral lead of the power utilityoutlet exceeds an established protective level.
 5. The protectivecircuit of claim 1, further including: a voltage threshold sensingcircuit for detecting when the voltage on the hot lead of the powerutility outlet exceeds an established protective level; a relay supplyswitch for providing current to the relay circuit; an electronic switchresponsive to the voltage threshold sensing circuit for disabling therelay supply switch and protecting the electrical apparatus when theconnection between the ground lead is disconnected.
 6. The protectivecircuit of claim 1, further including: a voltage threshold sensingcircuit for detecting when the voltage on the hot lead of the powerutility outlet exceeds an established protective level; a relay supplyswitch for providing current to the relay circuit; an electronic switchresponsive to the voltage threshold sensing circuit for disabling therelay supply switch and protecting the electrical apparatus when theconnection between the hot and neutral lead is reversed.
 7. Theprotective circuit of claim 1, wherein: said first relay receivescurrent when the hot lead and the neutral lead are properly connectedand not receiving current when connection between the hot lead and theneutral lead is reversed, said first relay switch arm opening whencurrent in the first relay circuit is not received to protect theelectrical apparatus and the voltage surge protectors of the voltagesurge protector circuit when the connection between the hot lead and theneutral lead is reversed.
 8. The protective circuit of claim 1, furtherincluding: a second relay connected between the hot and neutral leads ofthe protective circuit and controlling a second relay switch arm in aconductor connecting the hot lead of the power utility outlet to the hotlead of the electrical apparatus when the second relay is receivingcurrent, said second relay receiving current when the ground lead isconnected to an electrical ground, said second relay switch arm openingwhen current in the second relay circuit is not received to protect theelectrical apparatus and the voltage surge protectors of the voltagesurge protector circuit when the ground lead is not connected to anelectrical ground.
 9. The protective circuit of claim 1, furtherincluding: a second relay connected between the hot and neutral leads ofthe protective circuit and controlling a second relay switch arm in aconductor connecting the hot lead of the power utility outlet to the hotlead of the electrical apparatus when the second relay is receivingcurrent, said second relay receiving current when the hot lead and theneutral lead are properly connected and not receiving current whenconnection between the hot lead and the neutral lead is reversed, saidsecond relay switch arm opening when current in the second relay is notreceived to protect the electrical apparatus and the voltage surgeprotectors of the voltage surge protector circuit when connectionbetween the hot lead and the neutral lead is reversed.
 10. A protectivecircuit having hot, neutral, and ground leads arranged to be placedbetween corresponding hot, neutral, and ground leads of a power utilityoutlet and corresponding hot, neutral, and ground leads of electricalapparatus, said protective circuit comprising: a voltage surgeprotection circuit connected between the hot and neutral leads,comprising: first and second voltage surge protectors connected inseries between the hot and neutral leads, said first and second voltagesurge protectors being connected at a common connection to each other; afirst relay connected between the hot and neutral leads of theprotective circuit and controlling a first relay switch arm whichconnects the neutral lead of the power utility outlet and the neutrallead of the electrical apparatus when the first relay is receivingcurrent, said first relay receiving current when the hot lead and theneutral lead are properly connected and not receiving current whenconnection between the hot lead and the neutral lead is reversed, saidfirst relay switch arm opening when current in the first relay circuitis not received to protect the electrical apparatus and the voltagesurge protectors of the voltage surge protector circuit when theconnection between the hot lead and the neutral lead is reversed.